This invention relates to molten salt distillation to separate hafnium from zirconium and in particular relates to electrochemically preventing a buildup of iron chloride in molten salts (e.g. in phosphorus oxychloride complex of zirconium-hafnium chloride).
Molten (fused) salt electrochemical (electrolytic) processes for deposition of metal on one electrode (with evolution of chlorine gas at the other electrode) are known in the art. U.S. Pat. Nos. 3,764,493 to Nicks et al. and 4,670,121 to Ginatta et al. are examples of such processes.
Naturally occurring zirconium ores generally contain from 1 to 3 percent hafnium oxide relative to zirconium oxide. In order that the zirconium metal be acceptable as a nuclear reactor material, the hafnium content must first be reduced to low levels, due to the high neutron absorption cross section of hafnium. This separation process is difficult due to the extreme chemical similarity of the two elements. A number of techniques have been explored to accomplish this separation, with the technique currently in use in the United States involving liquid-liquid extraction of aqueous zirconyl chloride thiocyanate complex solution using methyl isobutyl ketone, generally as described in U.S. Pat. No. 2,938,679, issued to Overholser on May 31, 1960, with the removal of iron impurities prior to solvent extraction generally as described in U.S. Patent 3,006,719, issued to Miller on Oct. 31, 1961.
Several other processes have been suggested for separation of the zirconium-hafnium tetrachloride (Zr, Hf) Cl.sub.4 generated from the ore by carbochlorination. The use of a nonaqueous separation offers significant economic incentive over those processes requiring aqueous zirconium solutions. Direct distillation of the tetrachlorides provides one possible route, relying on the difference in boiling points between zirconium tetrachloride and hafnium tetrachloride. Unfortunately, direct distillation cannot be accomplished at near atmospheric pressure, since neither tetrachloride exhibits a liquid phase except at very high pressure. U.S. Pat. No. 2,852,446, issued to Bromberg on Sept. 16, 1958, describes a high pressure distillation process where the pressure, rather than a solvent, provides for a liquid phase.
U.S. Pat. No. 2,816,814 issued to Plucknett on Dec. 17, 1957, describes extractive distillation for separation of the tetrachlorides using a stannous chloride solvent. U.S. Pat. No. 2,928,722 to Scheller, issued Mar. 15, 1960, describes the batch fractional distillation of niobium and tantalum chlorides to separate these chlorides from each other and from other chloride impurities, and uses a "flux" to provide the molten salt phase, utilizing either zirconium tetrachloride-phosphorus oxychloride complex or an alkali metal chloride and aluminum (or iron, or zirconium) chloride mixture as the flux. U.S. Pat. No. 3,966,458 issued to Spink on June 29, 1976 provides a sodium-potassium chloride solvent for use in the extractive distillation of zirconium and hafnium tetrachlorides. U.S. Pat. No. 3,671,186 issued to Ishizuka on June 20, 1972 utilizes a series of dissolution and evaporation stages with a solvent such as sodium chloride. U.S. Pat. No. 4,021,531 issued to Besson on Apr. 3, 1977, utilizes extractive distillation with an alkali metal chloride and aluminum (or iron) chloride mixture as the solvent. Extractive distillation of zirconium-hafnium tetrachloride with a pure zinc chloride solvent has been attempted (Plucknett et al., AEC Report ISC-51, 1949), but was unsuccessful due to the formation of a highly viscous two-phase system. The anomalously high viscosity of zinc chloride is described by MacKenzie and Murphy (J. Chem. Phys., 33, 366, 1960). U.S. Pat. No. 4,737,244 to McLaughlin et al. describes an extractive distillation method for separating hafnium from zirconium of the type wherein a mixture of zirconium and hafnium tetrachlorides is introduced into a distillation column, with a recirculating molten salt solvent in the column to provide a liquid phase, and the improvement comprising having a molten salt solvent composition of at least 30 mole percent zinc chloride and at least 10 mole percent of lead chloride.
A process for zirconium-hafnium separation is described in U.S. Pat. No. 4,749,448 filed issued June 7, 1988 to Stoltz et al. This patent provides for zirconium-hafnium separation by extractive distillation with the molten solvent containing zinc chloride, it utilizes at least 80 mole percent zinc chloride, with the remainder including a viscosity reducer of magnesium chloride, calcium chloride, or mixtures thereof.
Of all of the molten salt distillation processes, only the above-mentioned Besson process with a potassium chloride-aluminum chloride solvent has been brought to commercial development. This process is currently in use in France and provides product zirconium tetrachloride, relatively depleted of hafnium tetrachloride in the liquid bottoms stream, and a hafnium tetrachloride enriched vapor stream taken from the top of the column. A relatively high reflux is provided by a condenser at the top of the column and a reboiler at the bottom of the column. Because of the stability of the double salts formed with the alkali metal chloride in the solvent, it is very difficult to completely separate the product zirconium tetrachloride from the solvent, and relatively high (e.g. 500.degree. C.) temperatures are required. Aluminum chloride in excess of 1:1 molar to alkali metal chloride is required and there is considerable carry-over of aluminum chloride into the zirconium tetrachloride leaving the stripper. French Pat. No. 2,543,162 (9-28-84) to Brun and Guerin describes a post-stripping process for removing aluminum chloride. In addition, it should be noted that aluminum chloride is an especially hygroscopic and corrosive molten salt, and, at higher temperatures, is very difficult to handle.
Another separation process involves fractionation of the chemical complex formed by the reaction of (Zr,Hf)Cl.sub.4 with phosphorus oxychloride (POCl.sub.3). This technique was patented in 1926 by van Arkel and de Boer (U.S. Pat. No. 1,582,860), and was based on the approximately 5.degree. C. boiling point difference between the hafnium and zirconium complex pseudoazeotropes, having the nominal compositions 3(Zr,Hf)Cl.sub.4 :2POCl.sub.3. This composition may be produced by direct reaction between liquid phosphorus oxychloride and the crude zirconium-hafnium tetrachloride obtained from opening of the ore.
Extensive work was done on the zirconium-hafnium tetrachloride complex with phosphorus oxychloride in the early 1950s, utilizing generally the molten salt distillation process of the aforementioned U.S. Pat. No. 1,582,860 of van Arkel and de Boer. While this extensive effort did provide some separation, the liquid in the reboiler became viscous during operation and both the reboiler liquid volatility and the Hf/Zr separation factor degraded significantly with time. Despite the extensive investment in time and money, this approach was abandoned and the U.S. effort was concentrated on the liquid-liquid extraction described in the above-mentioned U.S. Pat. No. 2,938,769 of Overholser. The liquid-liquid extraction remains the only commercially utilized process for zirconium-hafnium separation in the United States today.